1. Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to an isolation-enhanced system and method for a mobile station for a TDD-based mobile communication system.
2. Description of the Related Art
A time division synchronous CDMA (TD-SCDMA) is a third-generation mobile communication standard being developed by the initiative of China. The standard was approved by the ITU as a third generation mobile communication standard together with SCDMA and CDMA-2000 on November 1999.
The TD-SCDMA aims at increasing channel capacity and effectively managing resources by combining merits of time division duplex (TDD) and CDUA such as uplink synchronization and smart antenna.
In a TDD-based communication system such as the TD-SCDMA, transmission and reception channels are divided in time. Therefore, the receiving and transmitting modes of a mobile station are determined by the switching function of a transceiver. FIG. 1 is a block diagram illustrating an RF transceiver of a TD-SCDMA terminal. As shown, the transceiver switches between a reception or a transmission mode by operation of switch 3, controlled by a control signal.
In the reception mode, an RF signal received through antenna 1 is led to a receiving part of the transceiver such that the signal is amplified in a low-noise amplifier (LNA) 4, filtered by a filter 5, and then converted into a baseband signal in an intermediate frequency (IF) mixer. In the transmission mode, a transmission signal is up-converted in the IF mixer and is filtered by a transmission filter 8, amplified in an amplifier 7, and then transmitted over the air through isolator 6, switch 3, duplexer 2, and antenna 1.
Generally, in the mobile station for TD-SCDMA, a single pole double throw (SPDT) switch 3 is used for switching the transceiver between transmission and reception modes, and the isolator 6 is located right before the SPDT switch 3 on the transmission line to prevent a backflow of the transmission signal.
Unfortunately, in a conventional transceiver, there is a trade-off between isolation characteristics of the switch and an insertion loss for a receiving party, such that a switch with good isolation characteristics causes insertion loss for the receiving party and limits isolation performance of the commercial isolator.
In addition, if elements of a transmitting and receiving party are commonly grounded to a terminal frame with insufficient ground area, an undesired transmission signal can be induced to the receiving party through the ground line even though the undesired signal induction value depends on a PCB design. Since in power class A of the TD-SCDMA system, the maximum output power is +33 dBm and sensitivity level is −108 dBm. Therefore, the commercial SPDT switch is limited in fully isolating the transmission and reception channel.